Subject: Electron/Ion Beams 'Breaking' in a Gas Discharge Tube

In a gas discharge tube is simple air.  When high voltage AC is 
applied to this tube and the pressure is low enough the gases 
will glow, naturally.  At one point, though, the gas pressure is 
at a point where something very strange happens.  Instead of the 
usual beam of charged particles streaming across the tube, the 
beam of light has several large and quite distinct "breaks" 
every so often.

The pattern in which the beam of light is present/absent is:

A beam of light at the cathode, then an equal absence, then an 
equal beam of light, then an equal absence, then the center of 
the tube has an equal beam of light, then an equal absence, then 
an equal beam of light, then an equal absence, then an equal 
beam of light at the anode.

This phenomenon has remained a mystery to many a physics teacher 
and everyone who sees it occur.  What causes this strange 
"breaking" phenomenon in the gas discharge tube?

Re: Electron/Ion Beams 'Breaking' in a Gas Discharge Tube